Macromolecular coal structure as revealed by novel diffusion tests

Abstract

The main goal of the presentwork was the elucidation of the mechanisticcharacteristicsof dynamicmmsportof various penetrants (solvents) in thin sections of coals by examining their l:mnctrant uptake, front _,_vellingand stress development. The samples used were well char_cmfized, and the experimentation temperature varied from30 to 175°C.An important objective ofthis work was the study ofcoalnetwork sn-ucmre indifferent thermodynamically compatible penetranm and theanalysis of dynamicswelling in termsof present anomalous tz'amIx_ theories. Interferometry/polariscopy, surface image analysis and related techniques were used to quantify the stresses and solvent concentration profiles inthese sections. The sweUingc_te_tics ofselected coals wereelucidated by penetration studies usinga number of potentially swelling agents. Dynamic and equilibrium swelling behavior were correlated usingthepolar interaction con_butions of thesolvent solubility parameters. The penen'ant front position was followed inthin coal sections asa function ofdme. The initial front velocity was calculated forvarious coals and penetrants. Our penewant studies withthin coal sections fromthesamecoal sample butwith different thicknesses showthat within therange of150 _m to1500_xnthetransport mechanismofdimethyl formamide inthemacromolecular coal network is non-Fickian. Infact, for thethickest samples themmsportmechanism ispredominately Case-II whereas inthethinner samples pene, trant uptake may bediffusion-controlled. Studies invarious penetrants such asacetone, cyclohexane, methanol, methyl ethyl ketone, toluene andmethylene chloride indicated the'c penetrant transport isa non-Fickian phenomenon.However,theequilibrium swelling valueatroom temperann'e should be correlated withthepolar contribution ofthesolvent solubility parameter. The penetrant front velocity was correlated toC% (dmmf)and _[cof thevarious coals studied, and tothe solvent compatibility. Stresses and cracks wereobserved fortransport ofmethylene chloride. Inactual coalliquefaction and solvent treatment processes, thephenomenaobserved during thesolvent transport incoal and theswelling ofcoalchm_ksandpanicles arecharacterized by large stresses created inthe swelling system.Thesestresses aretheconsequence cff purely relaxational transport mechanism, and arcresponsible forsignificant cracking of thecoalsamples. In actual applications, this continuous crazing may lead tosignificantly increased reaction rates andmasstransfer ofsolvents incoal.